1. Field of the Invention
The present invention relates to a fire alarm system, and more particularly to a fire alarm system for determination of a fire presence by analysis of two different physical parameters associated with fire.
2. Description of the Prior Art
Japanese Patent Early Publication No. 4-270493 discloses a fire alarm system which monitors a smoke density and an ambient temperature as different parameters associated with fire, and determines a fire presence by analyzing the two monitored parameters. For this purpose, the system gives an inequity as a threshold which is defined by a function of a product of the smoke density and the ambient temperature, and determines the fire presence when the inequity is satisfied. Although this scheme of the fire determination is satisfactory for some environments, it is found still ineffective for the fire determination in a wide variety of environments having different possible sources of fires. That is, the prior system fails to recognize the fire presence when the fire occurs in a condition where either or both of the two parameters is relatively low. For example, the system cannot reliably recognize the fire caused by a non-flame smoldering accompanied with less amount of an initial temperature increase, and the fire caused by an alcohol burning accompanied with a low smoke density, particularly in a low temperature environment as in a winter.
The above insufficiency has been reduced in the present invention which provides an improved fire alarm system which is capable of reliably detecting the presence of fire caused by different sources. The fire alarm system in accordance with the present invention includes a smoke detector which detects a smoke density (S) in a target environment, and a temperature detector which detects a temperature (T) of the target space to provide a temperature difference (xcex94T) within a predetermined time interval. Included in the system is a threshold means which holds a plurality of primary criteria for determination of the fire presence. The primary criteria are:
(i) whether the smoke density (S) exceeds a first smoke threshold (S1) [e.g. Sxe2x89xa75%/m];
(ii) whether the temperature difference (xcex94T) exceeds a first temperature difference threshold (TD1) [e.g. xcex94Txe2x89xa718 C]; and
(iii) whether a combination of the smoke density (S) and the temperature difference (xcex94T) satisfies an inequality [e.g. 2S+xcex94Txe2x89xa712] which is based upon a decreasing function of xcex94T with an increase of S.
The system has a controller which checks the detected temperature difference xcex94T and the detected smoke density S with reference to the above primary criteria so as to provide a fire warning signal indicating a possible fire presence when anyone of the above primary criteria is satisfied.
Thus, by choosing suitable thresholds for the smoke density (S) and the temperature difference (xcex94T) and function of these parameters, it is possible to reliably detect the presence of fire occurring in a wide variety of environments. Particularly, by use of the temperature difference (xcex94T) as one criteria and as one variable combined with the smoke density (S) to constitute the function of the inequity, it is readily possible to give a consistent and reliable fire detection even at an early stage for the fire caused by various sources.
The first smoke threshold (S1) may be selected to be greater than the smoke density (S) given by the above function for a low range of the temperature difference (xcex94T) below a predetermined low limit (TDLow) which is lower than the first temperature difference threshold (TD1). Likewise, the first temperature difference threshold (TD1) may be selected to be greater than the temperature difference given by the above function for a low range of the smoke density (S) below a predetermined low limit (SLOW) which is lower than the first smoke threshold (S1). With the selection of the thresholds (S1, TD1), the system can successfully detect the fire characterized by a strong heat with less smoke density, e.g. the fire type of TF 6 (liquid fire  less than methylated spirits) as specified in the European Standards EU 54-9, and the smoldering characterized by a negligible heat increase but accompanied with a considerable amount of smoke density, e.g., the fire type TF-2 (smoldering pyrolysis  less than wood greater than ) and TF-3 (growing smoldering  less than cotton greater than ).
Preferably, the primary criteria may additionally include whether the temperature exceeds a first temperature threshold (T1) [e.g. Txe2x89xa757 C] for more reliable fire detection of fire characterized by a rapid growth of heat.
The controller is configured to check, at a regular time interval, whether or not anyone of the primary criteria is satisfied, and to have a fire decisive function in order to provide a reliable detection of a true fire presence. That is, upon occurrence of the fire warning signal, the fire decisive function operates to give a decision time period and issues the fire decisive signal indicative of the true fire presence when anyone of the primary criteria is satisfied continuously over the decision time period. Whereby, a reliable decision of fire can be made free from any possible errors due to a transient noise.
The controller is preferably given a weighing function of varying the decision time period according to which one of the primary criteria is relied upon to provide the fire warning signal so as to place a weight on determining the true fire presence, thereby reflecting different behaviors of the fire development due to different fire sources so as to achieve reliable decision of the true fire presence.
Further, in order to make the system more intelligent to learn and reflect the actual environment in which the detectors are mounted, the system is preferably designed to have different operation modes which give the decision time periods different from each other, while the threshold means is configured to hold stringent criteria which are analogous to the primary criteria but have low thresholds (S2, TD2) and function of inequality respectively different from those of the primary criteria. In this preferred version, the controller operates:
a) to check the detected temperature difference xcex94T and the detected smoke density S with reference to the stringent criteria, in order to provide a fire index indicating which one of the stringent criteria is satisfied by what number of such event within a past predetermined time duration, and
b) to select one of the different operation modes in accordance with the fire index in order to determine the true fire presence based upon the decision time period given to the selected mode.
Thus, the true fire decision can be made based upon different decision time period given to the selected mode reflecting the actual environment.
In detail, the system has a time table which specifies different ways of defining the time decision range in match with the environment so that the controller selects, from the time table, the way of defining the time decision range according to which one of the primary criteria is relied upon to provide the fire warning signal. At least one of the operation modes provided in the system is defined to modify the decision time period in a particular scheme. In this connection, the controller is configured to operate:
1) to check the detected temperature difference xcex94T and the detected smoke density S with reference to the stringent criteria, in order to provide a fire index indicating which one of the stringent criteria is satisfied by what number of such events within a past predetermined time range,
2) to select one of the different operation modes in accordance with the fire index,
3) to modify the way of the decision time period selected from the time table in accordance with the particular scheme of the selected operation mode, and
4) to determine the true fire presence based upon thus modified decision time period.
Thus, the true fire presence can be realized in a more sophisticated manner to be well reflective of the actual environment being learned by the system itself.
The particular scheme of modifying the decision time period when one of the primary criteria (i) and (iii) is satisfied, is defined, for example, by
a) sampling a plurality of the smoke densities (S) satisfying the one of the primary criteria over an immediately preceding time period;
b) obtaining amounts of thus sampled smoke densities (S) in excess of a smoke density level determined by the corresponding one of the primary criteria (i) and (iii);
c) summing the excess amount of the smoke densities (S); and
d) converting the summed amount into the decision time period.
Further, the threshold means may be designed to vary at least one of the first smoke threshold (S1) and the function of equality depending upon the operation mode selected.
The function of inequality utilized in the present invention may be a linear function expressed by xcex1xc2x7S+xcex94Txe2x89xa7xcex2, wherein xcex1 and xcex2 is a constant, for easy numerical processing.
These and still other objects and advantageous features of the present invention will become more apparent from the following description of the preferred embodiment when taken in conjunction with the attached drawings.